Systems and methods for measurement of vibration and motion of rotational devices are known. For example, many audio spectrum analyzers are available in the marketplace as well as modular approaches using analog-to-digital converter hardware.
The gas turbine engine is one rotational device that can benefit from this technology. Without positional information, many engine faults go unidentified or are identified incorrectly, until failure is imminent. Yet, due to extreme operating conditions, gas turbine engines are among the most difficult type of rotating machinery for engineering a synchronization solution.
The most common positioning solution for gas turbine engines is based on using a fixed reference point positioned on the shaft surface. However, this solution can be intrusive to normal turbine operation and can require shutdown conditions. Maintenance activities are then restricted only to those that can be performed when the unit is down (e.g. during a wash cycle). During shutdown conditions, dynamic testing is accomplished by applying a dab of reflective paint to mark a specific location on the shaft. A once-per-revolution signal is obtained by using a tachometer device connected to a borescope access port. Once the engine is started, however, the paint is reliable for only a short time as it loses its reflective characteristics soon after being subjected to the high temperatures and particulate matter passing through the engine. Use of the paint spot method also presents an operational limitation—the paint must typically be applied at least twenty-four hours prior to any subsequent testing. Typically, this prohibits normal turbine operation for thirty-six hours creating the potential for havoc for normal operations and severe financial losses.
In contrast to shutdown conditions, obtaining a clean once-per-revolution signal from the rotating shaft is the optimum method of gathering data of an operating gas turbine but poses significant engineering challenges. Some of these constraints include: (1) the probe cannot make contact with the shaft, (2) the shaft cannot be modified in any way, (3) nothing must be attached to the shaft, (4) the closest point to the shaft must be several inches away due to rotating compressor blades, (5) the shaft is fully enclosed in a pressurized section of the engine, where the nominal pressure can equal two-hundred pounds-per-square-inch (“PSI”), and (6) the shaft surface temperature can be approximately four-hundred degrees Fahrenheit.
Lacking accurate positional information during operation, many engine faults go unidentified until failure is imminent. While engine-monitoring technologies such as magnetic or radio frequency sensors can detect impending problems (e.g., engine vibration), they require special treatment or changes to the materials used in the machine construction. As a result, the fault remedy is global and not specific. Most often, the expeditious (but costly) remedy is replacement of the entire turbine, versus a time-consuming qualification of fault recognition, and subsequent repair of the causal condition.